Organotitanium Reagents in Organic Synthesis

Titanium has been used to perform many kinds of reactions in organic and inorganic chemistry. The present book is concerned primarily with a new development in titanium chemistry which is useful in organic synthesis. In 1979/80 it was discovered that the titanation of classical carbanions using C1TiX leads to species with reduced basicity and reactivity. This increases 3 chemo-, regio-and stereo selectivity in reactions with organic compounds such as aldehydes, ketones and alkyl halides. Many new examples have been reported in recent times. Since the nature of the ligand X at titanium can be widely varied, the electronic and steric nature of the reagents is easily controlled. This helps in predicting the stereochemical outcome of many of the C-C bond forming reactions, but the trial and error method is still necessary in other cases. One of the ultimate objectives of chemistry is to understand correlations between structure and reactivity. Although this goal has not been reached in the area of organotitanium chemistry, appreciable progress has been made. A great deal of physical and computational data of organotitanium compounds described in the current and older literature (e. g. , Ziegler-Natta type catalysts) has been reported by polymer, inorganic and theoretical chemists. It is summarized in Chapter 2 of this book, because some aspects are useful in understanding reactivity and selectivity of organo titanium compounds in organic synthesis as described in the chapters which follow.

Titanium has been used to perform many kinds of reactions in organic and inorganic chemistry. It is summarized in Chapter 2 of this book, because some aspects are useful in understanding reactivity and selectivity of organo titanium compounds in organic synthesis as described in the chapters which follow.

1. Introduction.- 1.1 Adjustment of Carbanion-Selectivity via Titanation.- 1.2 Other Uses of Titanium in Organic Chemistry.- References.- 2. Synthesis and Properties of Some Simple Organotitanium Compounds.- 2.1 Synthesis and Stability.- 2.2 Bond Energies.- 2.3 Bond Angles and Lengths.- 2.4 Aggregation State.- 2.5 Spectroscopic and Theoretical Aspects.- 2.6 Conclusions.- References.- 3. Chemoseleetivity in Reactions of Organotitanium Reagents with Carbonyl Compounds.- 3.1 Aldehyde/Ketone Differentiation.- 3.2 Aldehyde/Aldehyde and Ketone/Ketone Differentiation.- 3.3 Chemo- and Regioselective Additions to ?,?-Unsaturated Carbonyl Compounds.- 3.4 Aldehyde/Ester and Ketone/Ester Differentiation.- 3.5 Reactions in the Presence of Additional Functionality.- 3.6 Addition to Enolizable Ketones.- 3.7 Limitations of Organotitanium Reagents.- 3.8 Hints on How to Use Organotitanium Compounds.- 3.9 Why Does Titanation of Carbanions Increase Chemoselectivity?.- 3.10 Comparison with Other Organometallic Reagents.- 3.11 Reversal of Chemoselectivity: Chemoselective in situ Protection of Carbonyl Compounds.- 3.12 Organotitanium Reagents from Non-Organometallic Precursors 107 References.- References.- 4. Rates of Reactions.- 4.1 Kinetics of the Addition of CH3Ti(OCHMe2)3 to Carbonyl Compounds.- 4.2 Other Kinetic Studies.- References.- 5. Stereoselectivity in the Addition of Organotitanium Reagents to Carbonyl Compounds.- 5.1 Titanation of Carbanions as a Means to Control Stereoselectivity.- 5.2 Diastereofacial Selectivity.- 5.3 Simple Diastereoselectivity.- 5.4 The Problem of Equatorial vs. Axial Addition to Cyclic Ketones.- 5.5 Enantioselective Additions.- References.- 6. Michael Additions.- References.- 7. Substitution Reactions.- 7.1 Titanium Enolates as Nucleophiles.- 7.2Alkyltitanium Compounds as Nucleophiles.- 7.3 Other Substitution Reactions: Present and Future.- References.- 8. Wittig-type Methylenation of Carbonyl Compounds.- References.